[osx] Allow the user to configure a NSApplication subclass and use that instead.

Fixes http://hipbyte.myjetbrains.com/youtrack/issue/RM-539

test/test/spec/main_spec.rb

@@ -255,7 +255,7 @@ describe "NSNotFound" do
 end
 
 describe "Application singleton subclassing" do
-  xit "registers the principal class" do
+  it "registers the principal class" do
     app = (ios? ? UIApplication : NSApplication).sharedApplication
     app.class.should == ApplicationSubclass
   end